Motor vehicles are known that comprise a drive train with an automated friction clutch arranged between an internal combustion engine and a transmission. The transmission can for example be a manually shifted or automatically actuated helical gearbox or a double-clutch transmission with two friction clutches.
The automated friction clutch independently performs clutch engagements triggered by a control unit. The control unit applies to the clutch actuator a control variable that is to be adjusted and corresponds to the clutch torque and is to be transmitted via the friction clutch. The control variable is determined as the target torque in the control unit on the basis of inputs that are provided to the control unit as sensor data fed directly from other control units, such as a transmission control unit, or the driver. The target torque is assigned control variables by means of a characteristic curve, characteristic map, or calculation. To adapt target torques to the real clutch torque that is to be transmitted via the friction clutch, the sampling point and the real clutch torque, for example, are regularly determined to allow the characteristic curve of the target torque to be adapted, if necessary, to the real clutch torque by means of the control variable that is, for example, an axial path of the friction clutch, a rotational speed, or a rotational angle of an electric motor of the clutch actuator, or a switching variable of the electric motor and which can be made plausible by means of a corresponding sensor such as a position transducer or rotational angle sensor. The real clutch torque can vary over the short-term and long-term and is dependent, among other things, on the coefficient of friction of the friction linings, the friction clutch operating temperature and the like.
While the friction clutch is operating in an engaged state without slip, adaptation is not possible since the transmitted clutch torque is not observable in this operating state, that is, it cannot be detected by the corresponding sensor. Consequently, the target torque specified in normal operation when the friction clutch is engaged to maintain the engine torque and a safety margin to prevent slip is reduced at regular intervals between gear shifts, for example depending on the driving situation, until slip arises in the friction clutch. The clutch torque transmitted via the friction clutch can be calculated from the engine torque data and the slip calculated from the differential rotational speed between the crankshaft and the transmission input shaft, can then be adapted as the target torque.
Friction clutch slip phases that are periodically introduced in this manner for adapting the target torque cause additional wear on the friction clutch, especially on the friction linings of the clutch disc, and lead to greater fuel consumption. When the interval between the adaptation procedures is increased, impaired clutch engagement is observed which causes uncomfortable shifting, especially in conjunction with an automatically actuated transmission.
The problem therefore arises of proposing a method for controlling an automated friction clutch in which the target torque is adapted with sufficient precision to the actually transmitted clutch torque with reduced fuel consumption without regularly introducing slip phases.